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United States Patent |
5,006,304
|
Franklin
,   et al.
|
*
April 9, 1991
|
Pressure pulse cleaning method
Abstract
A method for loosening and removing sludge and debris from the vessel of a
heat exchanger, such as the secondary side of a nuclear steam generator,
is disclosed herein. The method generally comprises the steps of providing
a sufficient volume of a liquid, such as water, into the steam generator
so that the lower portion which includes the tubesheet is submerged, and
then generating a succession of pressure pulses within the water from one
or more pressure pulse generators wherein each pressure pulse creates
shock waves that exert momentary forces throughout the submerged portion
of the generator of a magnitude sufficient to loosen the sludge and
debris, but safely below the yield and fatigue limits of the heat
exchanger tubes and other components within the generator. The pressure
pulses commence as soon as a sufficient amount of water is introduced into
the steam generator to submerge the tubesheet, and continue all the way
through the draining of the steam generator.
Inventors:
|
Franklin; Richard D. (Jeannette, PA);
Auld; Gregg D. (Trafford, PA);
Murray; David E. (Greensburg, PA);
Lescisin; John J. (Pittsburgh, PA);
Vaia; Albert R. (Export, PA);
Dunn; Thomas E. (Pittsburgh, PA)
|
Assignee:
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Westinghouse Electric Corp. (Pittsburgh, PA)
|
[*] Notice: |
The portion of the term of this patent subsequent to May 1, 2007
has been disclaimed. |
Appl. No.:
|
424879 |
Filed:
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October 20, 1989 |
Current U.S. Class: |
376/316; 122/382 |
Intern'l Class: |
F22B 037/48 |
Field of Search: |
376/260,316,308,309,310
122/381,382,392,397
165/95,84
134/22.18,169 R
|
References Cited
U.S. Patent Documents
2604895 | Jul., 1952 | Fechter.
| |
3108418 | Apr., 1965 | MacLeod.
| |
3364983 | Jan., 1968 | Krinov et al.
| |
3409470 | Nov., 1968 | Karpovich.
| |
3457108 | Jul., 1969 | Hittel.
| |
4057034 | Nov., 1977 | Farquhar.
| |
4079701 | Mar., 1978 | Hickman et al.
| |
4492113 | Jan., 1985 | Weatherholt.
| |
4492186 | Jan., 1985 | Helm.
| |
4645542 | Feb., 1987 | Scharton et al.
| |
4655846 | Apr., 1987 | Scharton et al.
| |
4676201 | Jun., 1987 | Lahoda et al.
| |
4699665 | Oct., 1987 | Scharton et al.
| |
4715324 | Dec., 1987 | Muller et al.
| |
4756770 | Jul., 1988 | Weems et al.
| |
4773357 | Sep., 1988 | Scharton et al.
| |
4921662 | May., 1990 | Franklin et al. | 376/316.
|
Foreign Patent Documents |
132112 | Jan., 1985 | EP.
| |
133449 | Feb., 1985 | EP.
| |
1105895 | May., 1961 | DE.
| |
2568985 | Feb., 1986 | FR.
| |
48-89567 | Nov., 1973 | JP.
| |
575147 | Oct., 1977 | SU.
| |
597443 | Mar., 1978 | SU.
| |
Other References
"Proposal for Steam Generator Pressure Pulse/Sludge Lance at Farley", dated
Sep. 17, 1986.
"Pressure Pulse Cleaning in Westinghouse Recirculating Steam Generators
(Model 51)", dated Oct. 1986.
"Proposal to CECO/Byron Station Unit 1 for Steam Generator Sludge Lancing
and Optional Pressure Pulse Cleaning", dated Dec. 1, 1986.
|
Primary Examiner: Wasil; Daniel D.
Claims
We claim:
1. A method for loosening and removing sludge and debris from the interior
of a steam generator that contains one or more heat exchanger components
by means of a pressure pulse generator for generating shock waves that
exert momentary forces of between 1.5 and 35 ksi on the heat exchanger
components, comprising the steps of:
a. providing a sufficient amount of liquid in the steam generator to
submerge a portion of the interior thereof that includes some of said
sludge, debris and heat exchanger components, and
b. generate a succession of pressure pulses within the liquid by
introducing pulses of pressurized gas within said liquid by means of at
least one pressure pulse generator having an opening that communicates
with the interior of said vessel to create shock waves which exert
momentary pressures of no more than about 35 ksi on the heat exchanger
components to loosen said sludge and debris without exceeding the yield
strength of the heat exchanger components.
2. The method as defined in claim 1, wherein each pressure pulse generator
generates one pressure pulse between about every 1 to 15 seconds.
3. The method as defined in claim 1, wherein said succession of pressure
pulses lasts over 24 hours.
4. The method as defined in claim 1, wherein said vessel includes lower and
higher portions, and wherein said liquid is provided in said vessel by
filling said vessel over a selected period of time from said lower to said
higher portions, and wherein the generation of said succession of pressure
pulses commences when said vessel is filled to the extent to where said
lower portion is submerged.
5. The method as defined in claim 4, wherein said pulses continue as said
vessel is filled with liquid from said lower to said higher portions.
6. The method as defined in claim 1, wherein said vessel includes lower and
higher portions, and wherein said liquid is provided in said vessel by
filling said vessel over a selected period of time from said lower to
higher portions, and then by draining said liquid over a selected period
of time from said higher to said lower portions.
7. The method as defined in claim 6, wherein said succession of pressure
pulses continues as said liquid is drained from said higher to said lower
portion.
8. The method as defined in claim 1, further including the step of removing
ionic species from the liquid to remove dissolved debris from the interior
of the vessel.
9. The method as defined in claim 6, further including the step of
purifying the liquid as it is being drained from the vessel to remove
ionic species therefrom.
10. The method as defined in claim 13, further including the step of
filling another heat exchanger vessel with the purified liquid from the
first heat exchanger vessel while said first vessel is being drained.
11. The method as defined in claim 9, wherein said ionic species are
removed by recirculating said liquid through a demineralizer means.
12. The method as defined in claim 6, wherein said liquid is recirculated
for a selected period of time between the time said liquid fills said
vessel and the time that said liquid is drained from said vessel.
13. The method as defined in claim 1, further including the step of
flushing said heat exchanger vessel prior to the commencement of said
succession of pressure pulses to remove loose sludge and debris therefrom.
14. The method as defined in claim 1, further including the steps of
terminating said succession of pressure pulses, and then flushing said
heat exchanger vessel to remove loose sludge and debris therefrom.
15. The method as defined in claim 4, wherein the pressure pulse generator
generates pressure pulses by introducing pressurized gas into the liquid,
and wherein the pressure of the gas introduced into the liquid is
dependent upon the static pressure that the liquid exerts upon the opening
of the pressure pulse generator.
16. The method as defined in claim 1, wherein two pressure pulse generators
are positioned on opposite sides of the interior of the vessel, and
further comprising the step of generating pulses by said generators at
times asynchronously to control the location in the vessel where the shock
waves produced in the liquid impinge.
17. A method for loosening and removing sludge and debris from the interior
of the secondary side of nuclear steam generator that contains a plurality
of metallic heat exchanger tubes mounted in a tubesheet by means of a
pressure pulse generator for generating shock waves that exert momentary
forces of between about 1.5 and 25 ksi on the heat exchanger tubes,
comprising the steps of:
a. introducing a sufficient amount of water into said secondary side to
submerge said tubesheet and a portion of said heat exchanger tubes, and
b. generating a succession of pressure pulses within the water to create
shock waves which exert momentary pressures throughout the tubesheet and
submerged portions of said heat exchanger tubes of a magnitude of no more
than about 25 ksi to loosen said sludge and debris without exceeding the
yield strength of the heat exchanger tubes or causing significant metal
fatigue in said tubes.
18. The method as defined in claim 17, wherein the shock waves generated in
the water exert momentary pressures on said tubesheet and submerged
portions of said heat exchanger tubes no greater than between about 15 and
25 ksi.
19. The method as defined in claim 17, wherein the shock waves generated in
the water exert momentary pressures on said tubesheet and submerged
portions of said heat exchanger tubes no greater than about 17 and 23 ksi.
20. The method as defined in claim 17, wherein the shock waves generated in
the water exert momentary pressures on said tubesheet and submerged
portions of said heat exchanger tubes no greater than about 18 and 21 ksi.
21. A method for loosening and removing sludge, debris and dissolved matter
from the secondary side of a steam generator of the type containing a
plurality of heat exchanger tubes mounted in a tubesheet at one end and
supported along their length by a plurality of vertically spaced support
plates, comprising the steps of:
a. introducing a flow of water into the secondary side of the steam
generator;
b. commencing the generation of a plurality of pressure pulses in the water
in the secondary side of the steam generator when said water submerges
said tubesheet, wherein each of said pulses is generated by introducing
between 60 and 100 cubic inches of an inert gas into said water that is
pressurized to between about 350 and 450 pounds per square inch, wherein
said pressure pulses are generated at uniform time intervals of between
about 5 and 12 seconds;
c. continuing the flow of water into the secondary side of the steam
generator until the level of the water within the secondary side thereof
is sufficiently high to immerse all of the support plates therein;
d. continuing the generation of pressure pulses at uniform intervals at a
time between 5 and 12 seconds while the level of the water in the
secondary side is raised to immerse all of the support plates, wherein the
pressure of the pressurized gas used to generate the pressure pulses is
increased from between about 350 to 450 psi to between about 750 to 850
psi;
e. draining water out of the secondary side of the steam generator while
continuing to generate pulses at uniform intervals anywhere between about
5 and 12 seconds by lowering the level of the water in the secondary side
from the upper support plates down to a level which immerses only the
tubesheet, wherein the pressure of the gas used to generate the pressure
pulses is lowered as the level of the water is lowered from between about
750 to 850 psi to between about 350 to 450 psi,
wherein the succession of pressure pulses lasts from between 24 and 52
hours.
22. A method for loosening and removing sludge and debris from the
secondary side of a steam generator of the type containing a plurality of
heat exchanger tubes mounted in a tubesheet at one end and supported along
their length by a plurality of support plates, comprising the steps of:
a. providing a sufficient amount of water within the secondary side to
submerge at least said tubesheet and portions of said heat exchanger
tubes;
b. generating a succession of pressure pulses within the water form one or
more pressure pulse generators having openings that communicate with said
water in order to generate shock waves which exert momentary pressures of
no more than between about 1.5 and 35 Ksi on the heat exchanger components
to loosen said sludge and debris without exceeding the yield strength of
the heat exchanger tubes, and
c. maintaining said succession of pressure pulses for a time period between
24 hours and 35 hours.
23. The method defined in claim 22, further comprising the step of
recirculating the water through a recirculation system having a
demineralizer bed in order to remove dissolved ionic species in the water
while the level of the water is raised to immerse the upper support plates
and then lowered to immerse only the tubesheet.
24. The method defined in claim 22, wherein a plurality of pressure pulse
generators are used which are positioned uniformly around the
circumference of the secondary side of the steam generator, and wherein
said generators generate pulses synchronously.
25. The method defined in claim 22, wherein the secondary side of the steam
generator includes at least one pair of opposing sludge lance ports, and
wherein the pressure pulses are introduced through the opposing sludge
lance ports.
26. The method defined in claim 25, wherein the pressure pulses introduced
through opposing sludge lance ports are generated slightly asynchronously
with respect to one another in order to vary the point over the tubesheet
of the steam generator wherein the shock waves resulting from the opposing
pulses impinge upon one another.
27. The method defined in claim 22, wherein the water removed from the
secondary side of the steam generator as the water level is lowered from
the uppermost support plates to the tubesheet is used to fill the
secondary side of another steam generator.
28. The method defined in claim 22, wherein the succession of pressure
pulses continues from between about 36 to 52 hours.
29. The method defined in claim 22, wherein the succession of pressure
pulses continues from between about 46 to 52 hours.
30. A method for removing sludge, debris and other impurities from the
interiors of a plurality of heat exchanger vessels, comprising the steps
of:
a. introducing a liquid into the interior of a first heat exchanger vessel;
b. generating pressure pulses within said liquid to loosen, suspend and
dissolve said sludge, debris and other impurities wherein said pressure
pulses are generated at uniform true intervals of between about 5 and 12
seconds;
c. recirculating said liquid from said first heat exchanger through a
recirculation system located outside of said vessel that removes said
suspended and dissolved sludge, debris and other impurities while
continuing to generate pressure pulses within said liquid so that said
liquid is purified before being reintroduced into said first vessel, and
d. introducing at least some of the purified liquid produced by the
recirculation system into the interior of a second heat exchanger vessel
in order to simultaneously drain said first heat exchanger vessel while
executing step a. with respect to a second heat exchanger vessel,
wherein said succession of pulses lasts over 24 hours.
31. The method defined in claim 30, wherein said sludge, debris and
impurities are located at lower and higher portions of the interior of the
vessel, respectively, and wherein step b. commences when a sufficient
amount of liquid has been introduced in the vessel to immerse said lower
portion.
32. The method defined in claim 31, wherein liquid continues to be
introduced into said vessel until said higher portion of said vessel is
immersed therein.
33. The method defined in claim 32, wherein said pulses are continuously
generated as the level of the liquid rises to said higher portion.
34. The method defined in claim 33, wherein said pulses are continuously
generated in the liquid in the first vessel as said vessel is drained.
35. The method defined in claim 34, wherein the generation of said pulses
ceases when the level of said liquid drops too low to immerse said lower
portion of said vessel.
36. The method defined in claim 30, wherein said heat exchanger vessels are
steam generators, and said liquid is water.
37. The method defined in claim 32, wherein said recirculation system
induces a circumferential flow of liquid around the interior of the vessel
as said liquid is introduced into said vessel up to said higher portion
thereof in order to effectuate the suspension and discharge of said
sludge, debris and other impurities from said vessel.
38. The method defined in claim 32, wherein the heat exchanger vessels are
steam generators, and the liquid is water, and the net rate of introducing
recirculated water into the steam generator is between about 25 and 45
gallons per minute.
39. The method defined in claim 32, wherein the heat exchanger vessels are
steam generators, the liquid is water, and the net rate of draining water
from said generator is between about 15 and 35 gallons per minute.
40. The method defined in claim 32, further including the step of
recirculating said liquid in said heat exchanger vessel for a selected
period of time before executing drain and fill step d.
41. The method defined in claim 40, the heat exchanger vessels are steam
generators, the liquid is water, and said water is recirculated through
said steam generator and said recirculation system at a rate of between
about 40 and 60 gallons per minute.
42. The method defined in claim 40, wherein said pulses are continuously
generated during said selected period of time.
43. The method defined in claim 36, wherein said steam generator includes a
tubesheet within its lower portion, and the generation of said pressure
pulses commences when said water immerses said tubesheet.
44. The method defined in claim 37, wherein the heat exchanger vessels are
steam generators, the liquid is water and the higher portion of said
generator includes support plates for supporting heat exchanger tubes.
45. A method for loosening and removing sludge and debris from the interior
of the secondary side of nuclear steam generator that contains a plurality
of metallic heat exchanger tubes mounted in a tubesheet, comprising the
steps of:
a. introducing a sufficient amount of water into said secondary side to
submerge said tubesheet and a portion of said heat exchanger tubes, and
b. generating a succession of pressure pulses within the water to create
shock waves which exert momentary pressures throughout the tubesheet and
submerged portions of said heat exchanger tubes of a magnitude sufficient
to loosen said sludge and debris, but safely below a magnitude which would
either exceed the yield strength of heat exchanger tubes or cause
significant metal fatigue in said tubes,
wherein said pressure pulses are generated by pulse generators located on
opposite sides of said secondary side at asynchronous times to control the
location in the vessel where the resulting shock waves produced in the
water impinge.
Description
BACKGROUND OF THE INVENTION
This is a Continuation application of Ser. No. 07/183,874, filed Apr. 19,
1988, now U.S. Pat. No. 4,921,662.
This invention generally relates to methods for cleaning heat exchanger
vessels, and is specifically concerned with an improved pressure pulse
cleaning method for loosening and removing sludge and debris from the
secondary side of a nuclear steam generator.
Pressure pulse cleaning methods for cleaning the interior of the secondary
side of a nuclear steam generator are known in the prior art, and have
been disclosed and claimed in U.S. Pat. Nos. 4,655,846 and 4,699,665. The
purpose of these methods is to loosen and remove sludge and debris which
accumulates on the tubesheet, heat exchanger tubes and support plates
within the secondary side. In such methods, the secondary side of the
generator is first filled with water. Next, the outlet of a gas-operated
pressure pulse generator is placed into communication with the water. Such
communication may be implemented by a nozzle which may be formed from
either a straight section of pipe oriented horizontally over the tubesheet
of the generator, or a pipe having a 90 degree bend which is oriented
vertically over the tubesheet. Both of these prior art methods generally
teach generating pressure pulses within the water by emitting gas through
the nozzle that is pressurized to between 50 and 5000 pounds per square
inch. The pulses are repeated at a frequency of one per second, and the
succession of pulses may last anywhere from between 1 and 24 hours. The
pressure pulses create shock waves in the water surrounding the tubesheet,
the heat exchanger tubes and support plates within the secondary side of
the generator. These shock waves effectively loosen and remove sludge
deposits and other debris that accumulates within the secondary side over
protracted periods of time.
While the cleaning methods disclosed in these patents represent a major
advance in the state of the art, the applicants have found that there are
limitations associated with these methods which limit their usefulness in
cleaning nuclear steam generators. However, before these limitations may
be fully appreciated, some general background as to the structure,
operation and maintenance of nuclear steam generators is necessary.
In the secondary side of such steam generators, the vertically-oriented
legs of the U-shaped heat exchanger tubes extend through bores in a
plurality of horizontally-oriented support plates vertically spaced from
one another, while the bottom ends of these tubes are mounted within bores
located in the tubesheet. The relatively small annular spaces between
these heat exchanger tubes and the bores in the support plates and the
bores in the tubesheet are known in the art as "crevice regions." Such
crevice regions provide only a very limited flow path for the feed water
that circulates throughout the secondary side of the steam generator. The
consequent reduced flow of water through these crevice regions results in
a phenomenon known as "dry boiling" wherein the feed water is apt to boil
so rapidly that these regions can actually dry out for brief periods of
time before they are again immersed by the surrounding feed water. This
chronic drying-out of the crevice regions due to dry boiling causes
impurities dissolved in the water to precipitate out in these regions. The
precipitates ultimately create sludge and other debris which can obstruct
the flow of feed water in the secondary side of the generator to an extent
to where the steam output of the generator is seriously compromised.
Moreover, the presence of such sludges is known to promote stress
corrosion cracking in the heat exchanger tubes which, if not arrested,
will ultimately allow water from the primary side of the generator to
radioactively contaminate the water in the secondary side of the
generator.
To remove this sludge, many cleaning methods were used prior to the advent
of pressure pulse cleaning techniques. Examples of such prior art cleaning
methods include the application of ultrasonic waves to the water in the
steam generator to loosen such debris, and the use of a high-powered jet
of pressurized water to flush such debris out (known as "sludge lancing").
However, such techniques were only partially successful due to the
hardness of the magnitite deposits which form a major component of such
sludges, and the very limited accessibility of the crevice regions of the
steam generator.
Since its inception, pressure pulse cleaning has been a very promising way
in which to remove such stubborn deposits of sludges in such small spaces,
since the shock waves generated by the gas operated pressure pulse
operators are capable of applying a considerable loosening force to such
sludges. However, the applicants have found that the methods disclosed in
both U.S. Pat. Nos. 4,655,846 and 4,699,665 have fallen short of
fulfilling their promise in several material respects. For example,
research conducted by the applicants indicates that pressure pulses
generated by gas pressurized at the lower end of the 50 to 5000 psi range
are generally too weak to effectively dislodge significant amounts of such
crevice-region sludges. While pressure pulses generated by gas pressurized
at the upper end of to 50 to 5000 psi range would certainly be powerful
enough to loosen and remove the sludges, this same research indicates that
the shock waves resulting from such pulses are capable of generating
momentary forces that would jeopardize the integrity of the heat exchanger
tubes in the vicinity of the nozzle of the pressure pulse generator. Thus
the prior art does not specifically indicate what range of pressure is the
most effective. Still another shortcoming observed by the applicants was
the lack of any means to remove dissolved ionic species from the water
during such prior art cleaning processes. Such ionic species, if not
removed, are capable of precipitating out in the form of new sludges after
the termination of the pressure pulse cleaning process if no provision is
made to remove them. Additionally, applicants observed that if the fine
particulate matter that is dislodged from the crevice regions is not
removed from the water during the pressure pulse cleaning method, these
fine particles of sludge are capable of settling onto the tubesheet and
densely depositing themselves into the crevice regions between the
tubesheet and the legs of the heat exchanger tubes, thereby defeating one
of the purposes of the cleaning method. The applicants have further
observed that the usefulness of prior art pressure pulse cleaning
processes is limited by the one pulse per second frequency that these
methods teach. Specifically, the applicants have observed that the
relatively rapid pulse frequency taught in the prior art does not give the
nozzle and manifold of the pulse generator sufficient time to fill back
with water, and thus leaves pockets of shock-absorbing gas in the nozzle
of the pulse generator which limits the efficacy of later generated pulses
in generating sludge-loosening shock waves. Finally, the applicants have
observed that the maximum 24 hour time limit taught in U.S. Pat. Nos.
4,655,846 and 4,699,665 may not be sufficient to completely loosen and
remove all of the sludges and debris from the interior of the secondary
side of a typical steam generator.
Clearly, what is needed is an improved pressure pulse cleaning apparatus
which overcomes the limitations associated with prior art pressure pulse
cleaning methods and which is imminently practical for use in the
secondary side of a nuclear steam generators.
SUMMARY OF THE INVENTION
Generally speaking, the invention is a method for loosening and removing
sludge and debris from the interior of the vessel of a heat exchanger,
such as the secondary side of a nuclear steam generator, that overcomes
the limitations associated with the prior art. The method comprises the
steps of filling the secondary side with a sufficient volume of water so
that the tubesheet and portions of the heat exchanger tubes are completely
submerged therein, and then generating a succession of pressure pulses
within the water from one or more pressure pulse generators in order to
create shock waves of an optimum power level that exert momentary
pressures throughout the submerged portion of the secondary side of a
magnitude sufficient to effectively loosen the sludge and debris, but
insufficient to cause yielding or fatigue in the heat exchanger tubes and
other components within the secondary side. Applicants have found that
these momentary pressures can have a maximum magnitude of between 10 and
30 ksi, and are more preferably of a magnitude of between 15 and 25 ksi,
depending upon the condition of the heat exchanger tubes contained
therein.
The pressure pulse generators each preferably include an opening that
communicates with a lower portion of the secondary side of the steam
generator for introducing a pulse of compressed gas therein. In the
preferred method of the invention, each of the pressure pulses is
generated by discharging between 50 and 100 cubic inches of inert gas into
the water that is pressurized to between 200 and 1600 psi, depending upon
the level of the water within the secondary side. If the level of the
water is high enough to submerge only the tubesheet, the lower portion of
the heat exchanger tubes, and the outlet of the pulse generator, then the
gas is pressurized to between only about 200 and 600 psi. If the level of
the water is raised to submerge the upper support plates within the
secondary side, the pressure of the gas is raised to between 600 and 1600
psi in order to compensate for the diminishment of the shock waves
generated by the pulses as a result of the increase of the static pressure
of the water around the outlet of each of the pressure pulse generators.
The applicants have empirically observed that when pressure pulses are
generated by pressurized gas in accordance with the aforementioned
parameters, that the resulting shock waves are powerful enough to
effectively remove sludge and debris, yet the maximum magnitude of the
momentary pressure applied to the heat exchanger tubes in the vicinity of
the outlets of the pressure pulse generators is well below the 30 ksi
limit. Hence, the shock waves created by such pressure pulses do not
jeopardize the integrity of the heat exchanger tubes in the vicinity of
the outlet of each of the pressure pulse generators.
Each of the pressure pulse generators may generate one pressure pulse
between about every 5 to 15 seconds, and preferably between every 7 and 10
seconds. The applicants have empirically observed that when pressure
pulses are generated within the aformentioned frequency range, that the
nozzle and other components of the pressure pulse generator have time to
fill back up with water so that there are no residual pockets of gas in
the device that could significantly absorb the hydraulic shock waves
generated by the next release of pressurized gas. Additionally, the
succession of pressure pulses may last anywhere from between 16 and 56
hours, and preferably last between about 20 and 48 hours. The applicants
have observed that extending the succession of pressure pulses beyond 24
hours almost always has the effect of dislodging and removing significant
additional amounts of sludge and debris from the interior of the secondary
side.
In one preferred method of the invention, the secondary side of the steam
generator is gradually filled with water over a selected period of time
until the upper support plates are completely submerged. However, the
generation of pressure pulses preferably commences when the water level
submerges only the tubesheet, the lower portions of the heat exchanger
tubes, and the opening of the pulse generator and continues during the
filling of the secondary side up to a level beyond the upper support
plate. At the same time, the water within the secondary side is
recirculated through both a filtration unit to remove particulate matter
and a demineralizer bed to remove ionic species therefrom. The removal of
particulate matter during the cleaning process helps to prevent fine
particulate matter from settling in the tubesheet crevice regions. To
facilitate such particulate removal, a peripheral flow is induced in the
water in the secondary side during recirculation. The removal of the ionic
species prevents these chemicals from later precipitating out within the
interior of the secondary side after the termination of the cleaning
method. After the secondary side has been completely filled, the water
continues to be recirculated through the demineralizer bed for a selected
period of time, whereupon the water is gradually drained therefrom. The
succession of pressure pulses preferably continues during both the
recirculation and the draining steps.
Where the secondary sides of two or more nuclear steam generators are to be
cleaned in the same facility, the water drained from the first steam
generator cleaned is preferably used to fill a second steam generator.
This is feasible since the water being drained from the first generator
has been polished and filtered by the constant recirculation of this water
through both a filtration unit and a demineralizer bed. The direct
draining of such water from a first steam generator into a second steam
generator that also needs cleaning not only minimizes the time required to
clean both generators, but further conserves the amount of demineralized
and polished water necessary to implement such cleaning.
In implementing the method of the invention, two of the pressure pulse
generators are preferably positioned on opposite sides of the interior of
the secondary side. While the pulses are preferably generated
synchronously, they may also be generated asynchronously with respect to
one another so that they will impinge off-center with respect to the
tubesheet. The applicants believe that such off-center or asymmetrical
shock wave impingement geometry may facilitate the cleaning in instances
where it is not possible to mount the pressure pulsers in opposition to
one another.
BRIEF DESCRIPTION OF THE SEVERAL FIGURES
FIG. 1 is a perspective view of a Westinghousetype nuclear steam generator
with portions of the exterior walls removed so that the interiors of both
the primary and secondary sides may be seen;
FIG. 2 is a partial cross-sectional side view of the steam generator
illustrated in FIG. 1 along the line 2--2;
FIG. 3A is a cross-sectional plan view of the steam generator illustrated
in FIG. 2 along the line 3A--3A;
FIG. 3B is an enlarged view of the area circled in FIG. 3A;
FIG. 3C is a cross-sectional side view of the portion of the support plate
and heat exchanger tubing illustrated in FIG. 3B along the line 3C--3C;
FIG. 4A is a plan view of a portion of a different type of support plate
and tubing wherein trifoil broaching is used in lieu of circular bores;
FIG. 4B is a perspective view of the portion of the support plate and
tubing illustrated in FIG. 4A;
FIG. 5 is a cross-sectional side view of the steam generator illustrated in
FIG. 1 along the line 5--5;
FIG. 6A is an enlarged view of the circled portion of FIG. 5 along with a
schematized representation of the pressurized gas source used to power the
pressure pulse generator assemblies;
FIG. 6B is a cross-sectional side view of the air gun used in each of the
pressure pulse generator assemblies of the invention;
FIG. 7 is a plan view of the steam generator illustrated in FIG. 5 along
the line 7--7;
FIG. 8 is a schematic view of the recirculation system used to implement
the method of the invention;
FIG. 9 is a graph illustrating the diminishment over time of the pressure
of the gas within the pressure pulse generator after the pulse generator
assembly is fired, and
FIG. 10 is a graph illustrating the relationship between the maximum stress
experienced by the heat exchanger tubes in the steam generator, and the
location of these tubes with respect to the tubesheet.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
General Overview Of The Application Of The Invention
With reference now to FIGS. 1 and 2, wherein like numerals designate like
components throughout all of the several figures, the apparatus and method
of the invention are both particularly adapted for removing sludge which
accumulates within a nuclear steam generator 1. But before the application
of the invention can be fully appreciated, some understanding of the
general structure and maintenance problems associated with such steam
generators 1 is necessary.
Nuclear steam generators 1 generally include a primary side 3 and a
secondary side 5 which are hydraulically isolated from one another by a
tubesheet 7. The primary side 3 is bowl-shaped, and is divided into two,
hydraulically isolated halves by means of a divider plate 8. One of the
halves of the primary side includes a water inlet 9 for receiving hot,
radioactive water that has been circulated through the core barrel of a
nuclear reactor (not shown), while the other half includes a water outlet
13 for discharging this water back to the core barrel. This hot,
radioactive water circulates through the U-shaped heat exchanger tubes 22
contained within the secondary side 5 of the steam generator 1 from the
inlet half of the primary side 3 to the outlet half (see flow arrows). In
the art, the water-receiving half of the primary side 3 is called the
inlet channel head 15, while the water-discharging half is called the
outlet channel head 17.
The secondary side 5 of the steam generator 1 includes an elongated tube
bundle 20 formed from approximately 3500 U-shaped heat exchanger tubes 22.
Each of the heat exchanger tubes 22 includes a hot leg, a U-bend 26 at its
top, and a cold leg 28. The bottom end of the hot and cold legs 24, 28 of
each heat exchanger tube 22 is securely mounted within bores in the
tubesheet 7, and each of these legs terminates in an open end. The open
ends of all the hot legs 24 communicate with the inlet channel head 15,
while the open ends of all of the cold legs 28 communicate with the outlet
channel head 17. As will be better understood presently, heat from the
water in the primary side 3 circulating within the U-shaped heat exchanger
tubes 22 is transferred to nonradioactive feed water in the secondary side
5 of the generator 1 in order to generate nonradioactive steam.
With reference now to FIGS. 2, 3A, 3B and 3C, support plates 30 are
provided to securely mount and uniformly space the heat exchanger tubes 22
within the secondary side 5. Each of the support plates 30 includes a
plurality of bores 32 which are only slightly larger than the outer
diameter of the heat exchanger tubes 22 extending therethrough. To
facilitate a vertically-oriented circulation of the nonradioactive water
within the secondary side 5, a plurality of circulation ports 35 is also
provided in each of the support plates 30. Small annular spaces or
crevices 37 exist between the outer surface of the heat exchanger tubes
22, and the inner surface of the bores 32. Although not specifically shown
in any of the several figures, similar annular crevices 37 exist between
the lower ends of both the hot and cold legs 24 and 28 of each of the heat
exchanger tubes 22, and the bores of the tubesheet 7 in which they are
mounted. In some types of nuclear steam generators, the openings in the
support plates 30 are not circular, but instead are trifoil or
quatrefoil-shaped as is illustrated in FIGS. 4A and 4B. In such support
plates 30, the heat exchanger tubes 22 are supported along either three or
four equidistally spaced points around their circumferences. Because such
broached openings 38 leave relatively large gaps 40 at some points between
the heat exchanger tubes 22 and the support plate 30, there is no need for
separate circulation ports 34.
With reference back to FIGS. 1 and 2, the top portion of the secondary side
5 of the steam generator 1 includes a steam drying assembly 44 for
extracting the water out of the wet steam produced when the heat exchanger
tubes 22 boil the nonradioactive water within the secondary side 5. The
steam drying assembly 44 includes a primary separator bank 46 formed from
a battery of swirl vane separators, as well as a secondary separator bank
48 that includes a configuration of vanes that define a tortuous path for
moisture-laden steam to pass through. A steam outlet 49 is provided over
the steam drying assembly 44 for conducting dried steam to the blades of a
turbine coupled to an electrical generator. In the middle of the lower
portions of the secondary side 5, a tube wrapper 52 is provided between
the tube bundle 22 in the outer shell of the steam generator 1 in order to
provide a down comer path for water extracted from the wet steam that
rises through the steam drying assembly 44.
At the lower portion of the secondary side 5, a pair of opposing sludge
lance ports 53a, 53b are provided in some models of steam generators to
provide access for high pressure hoses that wash away much of the sludge
which accumulates over the top of the tubesheets 7 during the operation of
the generator 1. These opposing sludge lance ports 53a, 53b are typically
centrally aligned between the hot and cold legs 24 and 28 of each of the
heat exchanger tubes 22. It should be noted that in some steam generators,
the sludge lance ports are not oppositely disposed 180 degrees from one
another, but are only 90 degrees apart. Moreover, in other steam
generators, only one such sludge lance port is provided. In the steam
generator arts, the elongated areas between rows of tubes 22 on the
tubesheet 7 are known as tube lanes 54, while the relatively wider,
elongated area between the hot and cold legs of the most
centrally-disposed heat exchanger tubes 22 is known as the central tube
lane 55. These tube lines 54 are typically an inch or two wide in steam
generators whose tubes 27 are arranged in a square pitch, such as that
shown in FIGS. 3A, 3B and 3C. Narrower tube lanes 54 are present in steam
generators whose heat exchanger tubes 22 are arranged in a denser,
triangular pitch such as shown in FIGS. 4A and 4B.
During the operation of such steam generators 1, it has been observed that
the inability of secondaryside water to circulate as freely in the narrow
crevices 37 or gaps 40 between the heat exchanger tubes 22, and the
support plates 30 and tubesheets 7 can cause the nonradioactive water in
these regions to boil completely out of these small spaces, a phenomenon
which is known as "dry boiling." When such dry boiling occurs, any
impurities in the secondary side water are deposited in these narrow
crevices 37 or gaps 38. Such solid deposits tend to impede the already
limited circulation of secondary side water through these crevices 37 and
gaps 38 even more, thereby promoting even more dry boiling. This generates
even more deposits in these regions and is one of the primary mechanisms
for the generation of sludge which accumulates over the top of the
tubesheet 7. Often the deposits created by such dry boiling are formed
from relatively hard compounds of limited solubility, such as magnitite,
which tends to stubbornly lock itself in such small crevices 37 and gaps
38. These deposits have been known to wedge themselves so tightly in the
crevices 37 or gaps 38 between the heat exchanger tubes 22 and the bores
32 of the support plates 30 that the tube 22 can actually become dented at
this region.
The instant invention is both an apparatus and a method for dislodging and
loosening such deposits, sludge and debris and completely removing them
from the secondary side 5 of a steam generator 1.
Apparatus Of The Invention
With reference now to FIGS. 5, 6A, 6B, 7 and 8 the apparatus of the
invention generally comprises a pair of pressure pulse generator
assemblies 60a, 60b mounted in the two sludge lance ports 53a, 53b, in
combination with a recirculation system 114. Because both of these
generator assemblies 60a, 60b are identical in all respects, the following
description will be confined to generator assembly 60b in order to avoid
unnecessary prolixity.
With specific reference to FIGS. 6A and 6B, pulse generator assembly 60b
includes an air gun 62 for instantaneously releasing a volume of
pressurized gas, and a single port manifold 92 for directing this
pressurized gas into a generally tubular nozzle 111 which is aligned along
the central tube lane 55 of the steam generator 1. The air gun 62 includes
a firing cylinder 64 that contains a pulse flattener 65 which together are
dimensioned to store about 88 cubic inches of pressurized gas. Air gun 62
further includes a trigger cylinder 66 which stores approximately 10 cubic
inches of pressurized gas, and a plunger assembly 68 having an upper
piston 70 and a lower piston 72 interconnected by means of a common
connecting rod 74. The upper piston 70 can selectively open and close the
firing cylinder 64, and the lower piston 72 is reciprocally movable within
the trigger cylinder 66 as is indicated in phantom. The area of the lower
piston 72 that is acted on by pressurized gas in trigger cylinder 66 is
greater than the area of the upper piston 70 acted on by pressurized gas
in the cylinder 64. The connecting rod 74 of the plunger 68 includes a
centrally disposed bore 76 for conducting pressurized gas admitted into
the trigger cylinder 66 into the firing cylinder 64. The pulse flattener
65 also includes a gas conducting bore 77 that is about .50 inches in
diameter. Pressurized gas is admitted into the trigger cylinder 66 by
means of a coupling 78 of a gas line 80 that is connected to a pressurized
tank of nitrogen 84 by way of a commercially available pressure regulator
82. Gas conducting bores 86a and 86b are further provided in the walls of
the trigger cylinder 66 between a solenoid operated valve 88 and the
interior of the cylinder 66. The actuation of the solenoid operated valve
88 is controlled by means of an electronic firing circuit 90.
In operation, pressurized gas of anywhere between 200 and 1600 psi is
admitted into the trigger cylinder 66 by way of gas line 80. The pressure
that this gas applies to the face of the lower piston 72 of the plunger 68
causes the plunger 68 to assume the position illustrated in FIG. 6B,
wherein the upper piston 70 sealingly engages the bottom edge of the
firing cylinder 64. The sealing engagement between, the piston 70 and
firing cylinder 64 allows the firing cylinder 64 to be charged with
pressurized gas that is conducted from the trigger cylinder 66 by way of
bore 76 in the connecting rod 74, which in turn flows through the
gasconducting bore 77 in the pulse flattener 65. Such sealing engagement
between the upper piston 70 and the firing cylinder 64 will be maintained
throughout the entire charging period since the area of the lower piston
72 is larger than the area of the upper piston 70. After the firing
cylinder 64 has been completely charged with pressurized gas between 200
and 1600 psi, the pressure pulse generator 60b is actuated by firing
circuit 90, which opens solenoid valve 88 and exposes gas passages 86a and
86b to the ambient atmosphere. The resulting escape of pressurized gas
from the trigger cylinder 66 creates a disequilibrium in the pressures
acting upon the lower and upper pistons 70, 72 of the plunger 68, causing
it to assume the position illustrated in phantom in less than a
millisecond. When the air gun 62 is thus fired, 10 cubic inches of
pressurized gas are emitted around the 360 degree gap 91 between the lower
edge of the firing cylinder 64 and the upper edge of the trigger cylinder
66, while the remaining 77 cubic inches follows 2 or 3 milliseconds later
through the gas conducting bore 77 of the pulse flattener 65. The
two-stage emission of pressurized air out of firing cylinder 64 lowers the
peak amplitude of the resulting shock wave in the secondary side, thereby
advantageously lowering the peak stress experienced by the heat exchanger
tubes 22 in the vicinity of the nozzle 111. In the preferred embodiment,
air gun 62 is a PAR 600B air gun manufactured by Bolt Technology, Inc.,
located in Norwalk, Conn., and firing circuit 90 is a Model FC100
controller manufactured by the same corporate entity.
The single port manifold 92 completely encloses the circumferential gap 91
of the air gun 62 that vents the pressurized gas from the firing cylinder
64. Upper and lower mounting flanges 94a, 94b are provided which are
sealingly bolted to upper and lower mounting flanges 96a, 96b that
circumscribe the cylinders 64, 66 of the air gun 62. The manifold 92 has a
single outlet port 98 for directing the pulse of pressurized gas generated
by the air gun 62 into the nozzle 111. This port 98 terminates in a
mounting flange 100 which is bolted onto one of the annular shoulders 102
of a tubular spool piece 104. The other annular shoulder 107 of the spool
piece 104 is bolted around a circular port (not shown) of a mounting
flange 109. The spool piece 104 and outlet port 98 are sufficiently long
so that the body of the air gu 62 is spaced completely out of contact with
the shell of the steam generator 1. This is important, as such spacing
prevents the hard outer shell of the air gun 62 from vibrating against the
shell of the generator 1 when it is fired. In the preferred embodiment,
both the single port manifold 92 and spool piece 104 are formed from
stainless steel approximately 0.50 inches thick to insure adequate
strength. The mounting flange 109 is also preferably formed from 0.50
thick stainless steel, and has a series of bolt holes uniformly spaced
around its circumference which register with bolt receiving holes (not
shown) normally present around the sludge lance port 52b of the steam
generator 1. Hence, the pulse generator assembly 62b can be mounted onto
the secondary side 5 of the steam generator without the need for boring
special holes in the generator shell.
The nozzle 111 of the pressure pulse generator assembly 60b includes a
tubular body 112. One end of the tubular body 112 is circumferentially
welded around the port (not shown) of the mounting flange 109 so that all
of the compressed air emitted through the outlet port 98 of the single
port manifold 92 is directed through the nozzle 111. A
complete-penetration weld is used to insure adequate strength. The other
end of the tubular body 112 is welded onto a tip portion 113 which is
canted 30 degrees with respect to the upper surface of the tubesheet 7.
Because the 30 degree orientation of the tip portion 113 induces an
upwardly directed movement along the nozzle 111 when the pulse generator
60b is fired, a gusset 113.5 is provided between the tubular body 112 of
the nozzle and mounting flange 109. In the preferred embodiment, the body
112 of the nozzle 111 is formed from stainless steel about 0.50 thick,
having inner and outer diameters of 2.0 and 2.5 inches, respectively. The
nozzle 111 is preferably between 20 and 24 inches long, depending on the
model of steam generator 1. In all cases, the tip portion 113 should
extend beyond the tube wrapper 52. Finally, two vent holes 113.9 that are
0.25 inches in diameter and 1.0 inch apart are provided on the upper side
of the tubular body 112 of the nozzle 111 to expedite the refilling of the
nozzle 111 with water after each firing of the air gun 62 (as shown in
FIG. 7). The provision of such vent holes 113.9 does not divert any
significant portion of the air and water blast from the air gun 62
upwardly.
It has been found that a 30 degree downward inclination of the tip portion
113 is significantly more effective than either a straight, pipe-like
nozzle configuration that is horizontal with respect to the tubesheet 7,
or an elbow-like configuration where the tip 113 is vertically disposed
over the tubesheet 7. Applicant believes that the greater efficiency
associated with the 30 degree orientation of the nozzle tip 113 results
from the fact that the blast of water and pressurized air emitted through
the nozzle 111 obliquely hits a broad, near-center section of the
tubesheet 7, which in turn advantageously reflects the shock wave upwardly
toward the support plates 30 and over a broad cross-section of the
secondary side. This effect seems to be complemented by the simultaneous,
symmetrical blast of air and water from the pulse generator 60a located
180 degrees opposite from pulse generator 60b. The symmetrical and
centrally oriented impingement of the two shock waves seems to create a
uniform displacement of water in the upper portion of the secondary side
5, as may be best understood with reference to FIG. 5. This is an
important advantage as one of the primary cleaning mechanisms at work in
the upper regions of the secondary side 5 of the steam generator seems to
be the near instantaneous and uniform vertical displacement of the water
from .25 to 60a, 60b. Still another important advantage associated with
the oblique orientation of the blast of air and water is that the peak
stress on the heat exchanger tubes 22 in the vicinity of the tip 113 is
lowered. By contrast, if the nozzle tip 113 were directed completely
horizontally, no part of the blast would be widely reflected upwardly, and
the force of the air and water blast would act orthogonally on the nearest
tube 22. Similarly, if the blast were directed completely vertically
toward the tubesheet 7, the impact area of the blast against the tubesheet
would be narrower, and peak tube stresses would again be higher as the
blast would be more concentrated.
With reference now to FIGS. 6A, 7 and 8, the apparatus of the invention
further includes a recirculation system 114 that is interconnected with
the pressure pulse generator assembly 60b by inlet hose 115, a
suction-inlet hose 121a, and a suction hose 121b. As is best seen in FIG.
6A, inlet hose 115 extends through the circular mounting flange 109 of the
pressure pulse generator assembly 60b by way of a fitting 117. At its
distal end, the inlet hose 115 is aligned along the main tube lane 55
above nozzle 111 as is best seen in FIG. 7. At its proximal end, the inlet
hose 115 is connected to an inlet conduit 119b that is part of the
recirculation system 114. Suctioninlet hose 121a and suction hose 121b
likewise extend through the mounting flange 109 by way of fittings 123a,
123b. Inlet hose 115 is provided with a diverter valve 126a connected
thereto by a T-joint 126.1 for diverting incoming water into suction-inlet
hose 121a as shown. Suction-inlet hose 121a includes an isolation valve
126b as shown just below T-joint 126.2. When suction-inlet hose 121a is
used as a suction hose, valves 126a and 126b are closed and opened,
respectively. When suction-inlet hose 121b is used as an inlet hose,
valves 126a and 126b are opened and closed, respectively.
The distal ends of the hoses 121a, 121b lie on top of the tubesheet 7, and
are aligned along the circumference of the tubesheet 7 in opposite
directions, as may best be seen in FIG. 7. Such an alignment of the inlet
hose 115 and hoses 121a, 121b helps induce a circumferential flow of water
around the tubesheet 7 when hose 121a is used as an inlet hose by shutting
valve 126b and opening valve 126a. As will be discussed later, such a
circumferential flow advantageously helps to maintain loosened sludge in
suspension while the water in the secondary side is being recirculated
through the particulate filters 145 and 147 of the recirculation system
114. The proximal ends of each of the hoses 121a, 121b are connected to
the inlet ends of a T-joint 125. The outlet end of the T-joint 125 is in
turn connected to the inlet of a diaphragm pump 127 by way of conduit
125.5b. The use of a diaphragm-type pump 127 is preferred at this point in
the recirculation system 114 since the water withdrawn through the hoses
121a, 121b may have large particles of suspended sludge which, while
easily handled by a diaphragm-type pump, could damage or even destroy a
rotary or positive displacement-type pump.
FIG. 8 schematically illustrates the balance of the recirculation system
114. The suction-inlet hose 121a and suction hose 121b of each of the
pressure pulse generator assemblies 60a, 60b are ultimately connected to
the input of diaphragm pump 127. The output of the diaphragm pump 127 is
in turn serially connected to first a tranquilizer 129 and then a flow
meter 131. The tranquilizer 129 "evens out" the pulsations of water
created by the diaphragm pump 127 and thus allows the flow meter 131 to
display the average rate of the water flow out of the diaphragm pump 127.
The output of the flow meter 131 is connected to the inlet of a surge tank
135 via conduit 133. In the preferred embodiment, the surge tank 135 has
an approximately 300 gallon capacity The outlet of the surge tank 135 is
connected to the inlet of a flow pump 137 by way of a single conduit 139,
while the output of the pump 137 is connected to the inlet of a cyclone
separator 141 via conduit 143. In operation, the surge tank accumulates
the flow of water generated by the diaphragm pump 127 and smoothly
delivers this water to the inlet of the pump 137. The pump 137 in turn
generates a sufficient pressure head in the recirculating water so that a
substantial portion of the sludge suspended in the water will be
centrifugally flung out of the water as it flows through the cyclone
separator 141.
Located downstream of the cyclone separator 141 is a one to three micron
bag filter 145 that is serially connected to a one micron cartridge filter
147. These filters 145 and 147 remove any small particulate matter which
still might be suspended in the water after it passes through the cyclone
separator 141. Downstream of the filters 145 and 147 is a 500 gallon
supply tank 151. Supply tank 151 includes an outlet conduit 153 that leads
to the inlet of another flow pump 155. The outlet of the flow pump 155 is
in turn connected to the inlet of a dimineralizer bed 157. The purpose of
the flow pump 155 is to establish enough pressure in the water so that it
flows through the serially connected ion exchange columns (not shown) in
the demineralizer bed 157 at an acceptably rapid flow rate. The purpose of
the demineralizer bed 157 is to remove all ionic species from the water so
that they will have no opportunity to reenter the secondary side 5 of the
generator 1 and create new sludge deposits.
Located downsteam of the demineralizer bed 157 is a first T-joint 159 whose
inlet is connected to conduit 161 as shown. An isolation valve 160a and a
drain valve 160b are located downstream of the two outlets of the T-joint
159 as shown to allow the water used in the cleaning method to be drained
into the decontamination facility of the utility. Located downstream of
the T-joint 159 is another T-joint 163 whose inlet is also connected to
conduit 161 as shown. Diverter valves 165a and 165b are located downstream
of the outlet of T-joint 163 as indicated. Normally valve 165a is open and
valve 165b is closed. However, if one desires to fill a second steam
generator 1 with the filtered and polished water drained from a first
steam generator in order to expedite the pressure pulse cleaning method,
valves 165a and 165b can be partially closed and partially opened,
respectively. Flowmeters 167a, 167b are located downstream of the valves
165a and 165b so that an appropriate bifurcation of the flow from conduit
161 can be had to effect such a simultaneous drain-fill step.
Additionally, the conduit that valve 165b and flowmeter 167b are mounted
in terminates in a quick connect coupling 167.5. To expedite such a
simultaneous drain-fill step, valves 165a and 165b are mounted on a
wheeled cast (not shown) and conduit 161 is formed from a flexible hose to
form a portable coupling station 168. Downstream of the portable coupling
station 168, inlet conduit 161 terminates in the inlet of a T-joint 169
that bifurcates the inlet flow of water between inlet conduits 119a and
119b.
Water is supplied through the recirculation system 114 through deionized
water supply 170, which may be the deionized water reservoir of the
utility being serviced. Water supply 170 includes an outlet conduit 172
connected to the inlet of another flow pump 174. The outlet of the flow
pump 174 is connected to another conduit 176 whose outlet is in turn
connected to the supply tank 151. A check valve 178 is provided in conduit
176 to insure that water from the supply tank 151 cannot back up into the
deionized water reservoir 170.
Method Of The Invention
With reference now to FIGS. 5, 6A and 6B, the method of the invention is
generally implemented by the previously described pressure pulse generator
assemblies 60a, 60b in combination with the recirculation system 114.
However, before these components of the apparatus of the invention are
installed in and operated in a steam generator 1, several preliminary
steps are carried out. In the first of these steps, the relative condition
of the heat exchanger tubes 22 is preferably ascertained by an eddy
current or ultrasonic inspection of a type well known in the art. Such an
inspection will give the system operators information which they can use
to infer the maximum amount of momentary pressures that the tubes 22 of a
particular steam generator can safely withstand without any danger of
yielding or without undergoing significant metal fatigue In this regard,
applicants have observed that heat exchanger tubes 22 in moderately good
condition can withstand momentary pressures of up to approximately 19 ksi
without yielding or without incurring significant amounts of metal
fatigue. By contrast, it is anticipated that relatively old heat exchanger
tubes 22 whose walls have been significantly weakened by corrosion and
fretting may only be able to withstand only 15 ksi, while relatively new
tubes which are relatively free of the adverse affects of corrosion or
fretting may be able to withstand up to 30 ksi without any adverse
mechanical effects.
After the tubes 22 have been inspected by an eddy current or ultrasonic
probe to the extent necessary to ascertain the maximum amount of momentary
pressure they can safely withstand, the secondary side 5 of the steam
generator 1 is drained and all loose sludge that accumulates on top of the
tube sheet 7 is removed by known methods, such as flushing or by sludge
lancing. In the preferred embodiment, sludge lancing techniques such as
those disclosed and claimed in U.S. Pat. Nos. 4,079,701 and 4,676,201 are
used, each of which is owned by the Westinghouse Electric Corporation.
Generally speaking, such sludge lancing techniques involve the
installation of a movable water nozzle in the sludge lance ports 53a, 53b
in the secondary side 5 which washes the loose sludge out of the generator
1 by directing a high velocity stream of water down the tube lanes 54.
After all of the loose sludge on top of the tubesheet 7 has thus been
removed, the pressure pulse generator assemblies 60a, 60b are installed in
the sludge lance ports 53a, 53b in the positions illustrated in the FIGS.
6A and 7. Specifically, the tubular body 112 of the nozzle 111 of each of
the generator assemblies 60a, 60b is centrally aligned along the main tube
lane 55 in a horizontal position as shown so that the canted nozzle tip
113 assumes a 30 degree orientation with respect to the flat, horizontal
upper surface of the tubesheet 7. Next, the recirculation system 114 is
connected to each of the pulse generator assemblies 60a, 60b by coupling
the inlet hose 115 of each to the flexible inlet conduits 119a and 119b,
and the suction-inlet hose 121a and suction hose 121b of each to flexible
suction conduits 125.5a, 125.5b via the T-joint 125 of each assembly 60a,
60b. Next, the recirculation system 114 is connected via conduit 172 to
the supply 170 of deionized water from the utility, as is best seen in
FIG. 8. The flow pump 174 is then actuated in order to fill supply tank
151 approximately one-half full, which will occur when tank 151 receives
about 250 gallons of water.
Once supply tank 151 is at least one-half full, flow pump 155 is actuated
to commence the fill cycle. In the preferred method of the invention, pump
155 generates a flow of purified water of approximately 120 gallons per
minute which is bifurcated to two 60 gallon per minute flows at T-joint
169 between inlet hose 119a and 119b on opposing sides of the generator 1
in order to fill the secondary side 5 of the steam generator 1. During the
time that the secondary side 5 is being filled via pump 153, valves 165a
and 165b are opened and closed so that the entire flow of water from pump
153 enters the generator 1. Additionally, valves 126a, 126b are opened and
closed in each of the generator assemblies 60a, 60b in order to further
bifurcate the 60 gallon per minute flow from inlet conduit 119a, 119b
between the inlet hose 115 and the suction-inlet hose 121a of each of the
generator assemblies 60a, 60b. As soon as the water level on the secondary
side 5 becomes great enough to submerge both hoses 121a, 121b diaphragm
pump 127 is actuated and adjusted to withdraw 50 gallons per minute a
piece out of the secondary side 5. Since the flow pump 155 introduces 120
gallons per minute, while the diaphragm pump 127 withdraws 50 gallons per
minute, the secondary side 5 is filled at a net flow rate of 70 gallons
per minute. Additionally, since the suction-inlet hose 121b of each of the
generator assemblies 60a, 60b is used at this time as a fill hose, whose
output is circumferentially directed toward an opposing suction hose 121a,
a peripheral flow of water is created around the circumference of the
secondary side as is best seen in FIG. 7. Such a peripheral flow of water
is believed to help keep in suspension the relatively large amounts of
sludge and debris that are initially dislodged from the interior of the
secondary side 5 when the generator assemblies 60a, 60b are actuated which
in turn allows the recirculation system 114 to remove the maximum amount
of dislodged sludge and debris during the fill cycle of the method.
After the water level in the secondary side 5 of the generator 1 rises to a
level of at least six inches over the nozzles 111 of each of the pressure
pulse generator assemblies 60a, 60b, the firing of the air gun 62 of each
of the assemblies 60a, 60b commences. If the prior eddy current and
ultrasonic testing indicates that the heat exchanger tubes 22 can
withstand momentary pressures of approximately 19 ksi without any
deleterious affects, the gas pressure regulators 82 of each of the
generator assemblies 60a, 60b is adjusted so that gas of a pressure of
about 400 psi is initially admitted into the firing cylinders 64 of the
air gun 62 of each. Such a gas pressure applies a peak stress to the tubes
22 which is safely below the 19 ksi limit, as will be discussed in more
detail hereinafter. The firing circuit 90 is then adjusted to fire the
solenoid operated valve 88 of the trigger cylinder 66 every seven to ten
seconds. The firing of the air gun 62 at seven to ten second intervals
continues during the entire fill, recirculation and drain cycles of the
method. While the generator assemblies 60a, 60b are capable of firing at
shorter time intervals, a pulse firing frequency of seven to ten seconds
is preferred because it gives the nitrogen gas emitted by the nozzle 111
sufficient time to clear the nozzle 111 and manifold 92 before the next
pulse. If pockets of gas remain in the pulse generator 60b during
subsequent air gun firings, then a significant amount of the shock to the
water within the secondary side 5 would be absorbed by such bubbles,
thereby interfering with the cleaning action.
It is important to note that the gas pressure initially selected for use
with the pressure pulse generator assembly 60a, 60b induces momentary
pressures that are well below the maximum safe amount of momentary forces
that the tubes 22 can actually withstand, for two reasons. First, as will
be discussed in more detail hereinafter, the pressure of the gas used in
the generator assembly 60a, 60b is slowly raised in proportion with the
extent to which the secondary side 5 of the steam generator 1 is filled
until it is approximately twice as great as the initially chosen value for
gas pressure. Hence, when the initial gas pressure used when the water
level is just above the nozzles 111 is 400 psi, the final pressure of the
gas used in the pressure pulse generator assembly 60a, 60b will be 800 to
900 psi. Secondly, the gas pressure is chosen so that the maximum pressure
used will induce momentary forces in the tubes 22 which are at least 30
and preferably 40 percent below the maximum ksi indicated by the
previously mentioned eddy current and ultrasonic inspection to provide a
wide margin of safety. In making the selection of which gas pressures to
use, applicants have discovered that there is a surprising, non-linear
relationship between the pressure of the gas used in the air gun 62 of
each pulse generator assembly 60a, 60b and the resulting peak stress on
the tubes 22 as is evident from the following test results:
______________________________________
Gas Pressure Peak Tube Stress
______________________________________
400 psi 5,580 psi
800 psi 12,090 psi
1600 psi 30,960 psi
______________________________________
In most circumstances, the firing of the air gun 62 of both the pulse
generators will be synchronous in order to uniformly displace the water
throughout the entire cross-section of the secondary side 5 of the
generator 1. However, there may be instances where an asynchronous firing
of the air guns 62 of the different assemblies may be desirable, such as
in a steam generator where the sludge lance ports 53a, 53b are only 90
degrees apart from one another. In such a case, the asynchronous firing of
the air guns 62 could possibly help to compensate for the non-opposing
arrangement of the pulse generators 60a, 60b in the secondary side 5
imposed by the location of the 90 degree apart sludge lance ports 53a,
53b.
FIG. 9 illustrates how the pressure of the gas within the 88 cubic inch
firing cylinder 64 of the air gun 62 diminishes over time, and FIG. 10
indicates the peak stress experienced by the column of tubes closest to
the nozzle 111. Specifically, when the pressure of the gas within the
firing cylinder 64 is 875 psi, and a 10 cubic inch pulse flattener 65
having a gas-conducting bore 0.50 inches in diameter is used, the gas
leaves the cylinder 62 over a time period of approximately five
milliseconds. FIG. 10 shows that the peak stress experienced by the column
of tubes 22 closest to the tip portion 113 of the nozzle 111 is between 12
and 13 ksi, which again is safely below the 19 ksi limit. If no pulse
flattener 65 were used, the closest column of heat exchanger tubes 22 in
the secondary side 5 to the tip portion 113 of the nozzle 111 would be
considerably higher, as the gas would escape from the air gun in a
considerably shorter time than 5 milliseconds.
The filling of the secondary side 5 at a net rate of 70 gallons per minute
continues until the uppermost support plate 30 is immersed with water. In
a typical Westinghouse Model 51 steam generator, about 17,000 gallons of
water must be introduced into the secondary side 5 before the water
reaches such a level. At a net fill rate of 70 gallons per minute, the
fill cycle takes about four hours. During the fill cycle, the pressure of
the gas introduced into the firing cylinder 64 of each air gun 62 is
raised from approximately 400 psi to approximately 800 to 900 psi in
direct proportion with the water level in the secondary side 5. The
proportional increase in the pressure of the gas used in the air guns 62
substantially offsets the diminishment in the power of the pulses created
thereby caused by the increasing static water pressure around the tip
portion 113 of the nozzle 111 of each.
As soon as the water level in the secondary side 5 is high enough to
completely submerge the highest support plate 30, the recirculation cycle
commences. If desired, valves 126a, 126b may be closed and opened,
respectively, in order to convert the function of suction-fill hose 121a
into a suction hose. Moreover, the flow rate of fill pump 155 is lowered
from 120 gallons per minute to only 50 gallons per minute, while the
withdrawal rate of the diaphragm type suction pump 127 is maintained at 50
gallons per minute. The net result of these adjustments is that water is
recirculated through the secondary side 5 of the steam generator 1 at a
rate of approximately 50 gallons per minute. This circulation rate is
maintained for approximately 12-48 hours while the air guns 62 of each of
the generator assemblies 60a, 60b are fired at a pressure of 800 psi every
seven to ten seconds.
After the termination of the recirculation cycle, the drain cycle of the
method commences. This step is implemented by doubling the flow rate of
the diaphragmtype suction pump 127 so that each of the hoses 121a, 121b of
each pulse generator 60a, 60b will withdraw approximately 22.5 gallons per
minute. Since the fill pump 155 continues to fill the secondary side 5 at
a total rate of approximately 50 gallons per minute, the net drain rate is
approximately 40 gallons per minute. As the secondary side 5 has about
17,000 gallons of water in it at the end of the recirculation cycle, the
drain cycle takes about seven hours. During this period of time, it should
be noted that the pressure of the gas introduced into the firing cylinders
64 of the air guns 62 of the generator assembly 60a, 60b is lowered from
800 psi to 400 psi in proportion with the level of the water in the
secondary side 5.
To expedite the cleaning method in a utility where two or more steam
generators are to be cleaned, a second steam generator (not shown) may be
filled with the filtered and polished water that flows out of the
demineralizer 157 of the recirculation system 114 during the drain cycle
of a first steam generator. This may be accomplished by wheeling the
portable coupling station 168 over to a second generator where other pulse
generator assemblies 60a, 60b have been installed, and coupling the outlet
of flowmeter 167b to the inlet conduits 119a, 119b of the second
generator. Next, diverter valves 165a and 165b are adjusted so that part
of the filtered and polished water leaving the demineralizer 157 is
shunted to the inlet conduits 119a, 119b of the second generator. In order
to maintain the seven hour time period of the drain cycle for the first
steam generator, the flow rate of the pump 155 is increased to
approximately 170 gallons per minute. The valve 165a is adjusted so that
the flow rate as indicated by flowmeter 167a remains approximately 50
gallons per minute. The balance of the 120 gallon per minute flow is
shunted through valve 165b to the secondary side 5 of the second steam
generator. The implementation of this additional step not only lowers the
total amount of time required to clean a plurality of steam generators by
as much as 50 percent, but further considerably reduces the amount of
deionized and purified water that the utility must supply from source 170
to implement the cleaning method of the invention. As it requires
approximately 17,000 gallons or 72 tons of water to clean a single steam
generator 1, the savings in water alone are clearly significant. Moreover,
by reducing the overall amount of time required to clean two generators,
the amount of time that the operating personnel are exposed to potentially
harmful radiation is considerably reduced. The portability of the valves
165a, 165b afforded by the portable conduit coupling station 168 plus the
use of a flexible hose for conduit 161 greatly facilitates the
implementation of such a combined drain-fill step in the method of the
invention.
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